UX Audit - Improve the trait input field when creating segment definitions

The trait field:


Should be better/more intelligent. It should be a combined autocomplete/dropdown/free text element.

## Populating the drop down

With the global API this is going to be tricky as we don't store a canonical list of trait keys. A 99% solution would be to grab ~50 identities at random and create a list of distinct trait keys from that data. This would be a significant improvement over the current implementation.

We would need to create an API endpoint to return this list.